Young calves which are put to pasture for the first time are susceptible to a large number of germs, i.e. threadlike small parasitic nematodes (Trichostrongylidae), which are parasites in the abomasum and intestine of cattle. These infections can cause serious diseases, such as diarrhea, indigestion, loss of weight, and death. But it is in particular the relatively mild infections which are of economic importance, since they are very widespread and result in poor growth, in particular at the end of the season.
Infected calves secrete the very small eggs of the nematodes in their faeces. FIG. 1 shows the life cycle of cattle nematode parasites. The mature parasites multiply by laying eggs in the intestinal tract of the cattle. The eggs have a size of 0.05-0.1 mm and are thin shelled. They are excreted with the dung of the animals. The larvae develop from the eggs in the cow pats on the pasture. The larvae go through three larval stages (L.sub.1, L.sub.2 and L.sub.3). The third larval stage (L.sub.3), which is infective, retain the cuticle of the second larval stage (L.sub.2) as a protective sheath until they enter their host. In the cow pat, temperature, moisture, oxygen pressure and the natural enemies of the parasites are decisive for the number of eggs that develop to the infective stage. The infective larvae, having a length of up to 1 mm, are spread from the cow pat to the surrounding grass, in particular in connection with rainy weather. Most of the larvae are present within 30 cm from the rim of the cow pats. On the grass they may be eaten by grazing animals, and then they can continue their development in the intestinal tract of the animals up to maturity.
The parasitic development starts when L.sub.3 larvae are ingested by cattle. L.sub.3 exsheath in the rumen. A few days after the infection, exsheathed L.sub.3 enter the abomasal glands, where they differentiate and increase in size. Three stages occur (L.sub.3, L.sub.4 and L.sub.5), separated by two moults. Immature adult parasites (L.sub.5) emerge from the glands around 18 days after the infection. During the next few days, they become sexually mature on the surface of the abomasal mucosa, resulting in excretion of eggs.
Since nematode parasites often cause great losses to farmers, it is very much of interest to control these parasites. Traditionally, this takes place by medical treatment, but farmers also try in various ways to protect grazing animals against eating large amount of larvae, e.g. by reducing the density of animals on the pasture so that they are not forced to graze the highly infected areas close to their own excrements. Animals can also be moved to a clean or low-infected field at the end of the summer when the danger of infection usually increases strongly.
However, these measures have a number of economic and practical drawbacks, making it desirable to find alternative methods of control. Thus, for a number of years efforts have been devoted to the development of methods for biological control of larvae of nematodes in cow pats already before they are spread as infective larvae to the grass. Biological control comprises the use of natural enemies of the nematodes, which are nematophagous fungi, also called predacious fungi, in this case.
Predacious fungi are microfungi which do not develop fruiting bodies, as is known from e.g. mushrooms, but have a growth form resembling mold. Predacious fungi are special having developed organs that are able to capture and kill small nematodes, including infective larvae of nematodes. Predacious fungi are originally terricolous fungi, but it has been found that they can also grow in cow pats. It is precisely here their beneficial effect should be used to kill large numbers of the infective larvae of parasitic intestinal worms.
FIG. 2 shows a nematode larva captured by a predatory fungus. The larva is captured in a sticky net consisting of three strong arcs. Where there is contact, the parasite is penetrated. Fungal hyphae extend from here to grow out and fill the body of the nematode which is eventually killed. Then the inner organs are dissolved and absorbed. The fungal mycelium inside the parasite is shown by dotted lines.
To utilize predacious fungi in practice it is necessary to select fungal species which can pass through the intestinal tract of cattle alive. Moreover, methods should be developed to produce large amounts of fungal material. It will hereby be possible to place predatory fungi in cow pats in the period during the grazing season when many parasite nematode larvae develop in cow pats.
So far all attempts to isolate efficient fungi which are able to pass the intestinal tract of animals, and thereby usable in biological control, have failed. It has also not been possible by any means to coat the fungal material in order to protect said material against exposure to the enzymes in the intestinal tract.
Since the discovery of nematophagous fungi about one hundred years ago, several attempts have been made at using these organisms as biological control agents against parasitic nematodes causing diseases in plants and animals. The nematophagous fungi capable of destroying free living nematodes can be divided into two groups, predaclous and endoparasitic (G. L. Barron; The nematode-destroying fungi; Topics in Mycobiology No. 1, Canadian Biological Publications Ltd., Canada (1977)). J. Gronvold et al., (1987), Journal of Helminthology, 61:65-71 (1987); ibid., 62:271-280 (1988); ibid., 63:115-126 (1989), have shown that the predacious fungus Arthrobotrys oligospora is capable of reducing the number of infective larvae (L.sub.3) of the bovine trichostrongyles Cooperia oncophora and Ostertegia ostertagi in dung and herbage. Arthrobotrys oligospora is one of the species most investigated in attempts of bio-control (J. Gronvold et al., Journal of Helminthology, 69;119-125 (1985); P. Nansen et al., Veterinary Parasltology, 26:329-337 (1988)), but it is not necessarily the most efficient one.
All previous attempts have failed because of the lack of nematophagous fungi, Including Arthrobotrys oligospora, which are able to pass through the intestinal tract of the animal and subsequently reduce the Infection of the herbage effectively.
M. Peloille, IOBC/WPRS Bulletin XIV/2 (1991) has studied the criteria which should be taken into consideration when selecting effective predacious fungi. These criteria are growth rate at different temperatures, predacious activity, ability to survive passage through the intestinal tract of animals, end ability to produce chlamydospores which can be used for the dispersal of the fungi. Duddingtonia flagrans is mentioned as the species which fulfills all the requirements best. The conclusion which is not based upon scientifically correct experiments is very misleading, because only few of the Duddingtonia fungi actually survive the passage through the Intestinal tract of ruminants.
One reason that the experiments are not scientifically correct is that it is not documented that the experimental and the control animals were comparable with respect to the number of parasite eggs. Furthermore, the animals have only been fed once with a very large dose of the predatory fungus (500 g), end even though this passes through the intestinal tract of the animals within 48 hours, the predatory fungus is claimed to be effective for up to 96 hours. Furthermore, there is no reference sample taken at the beginning of the experiment, since the results simply begin after 24 hours. It is. also Important to mention that the author bases her conclusion solely on pure laboratory experiments. It is not documented that the effect of the fungus Is maintained in field experiments.
These defects ere remedied by the selection technique according to the invention, which will be described more fully below.